\[\boxed{\text{941.\ }\text{еуроки}\text{-}\text{ответы}\text{\ }\text{на}\text{\ }\text{пятёрку}}\]
\[\textbf{а)}\ 2x - 1 > 0\]
\[2x > 1\]
\[x > \frac{1}{2}\]
\[при\ \ \ x \in (0,5;\ + \infty)\]
\[\textbf{б)}\ 21 - 3y < 0\]
\[- 3y < - 21\]
\[y > 7\]
\[при\ \ y \in (7;\ + \infty)\]
\[\textbf{в)}\ 5 - 3c > 80\]
\[- 3c > 80 - 5\]
\[- 3c > 75\]
\[c < - \frac{75}{3}\]
\[c < - 25\]
\[при\ \ c \in ( - \infty;\ - 25)\]